We describe a 10-day-old male presenting with drop foot and immobility in Ills right leg; no abnormality in physical examination had been found upon delivery or discharge. Magnetic resonance imaging showed a pelvic mass with an extension to the great sciatic foramen. Fine-needle aspiration biopsy revealed congenital infantile fibrosarcoma. He was started on vincristine, actinomycin-D, cyclophosphamide chemotherapy. However, lie died soon after the second cycle due to veno-occlusive disease of the liver. Differential diagnosis of drop foot and immobility in lower extremity must include infiltrating neoplasms in pelvis.